1. Field of the Invention
The present invention relates to a broad-band light source utilizing a semiconductor optical amplifier.
2. Description of the Related Art
Generally, broad-band light sources include a light emitting diode (LED), a super-luminescent diode (SLED), erbium-doped fiber amplifier (EDFA), etc. In particular, the EDFA widely used since it is capable of obtaining the polarization insensitive light of a high power stably. However, since a wavelength range of the light emitted from the EDFA is limited, the EDFA has a limited use as a broad-band light source. Further, the EDFA is larger than most semiconductor devices available, thus it is difficult to reduce the production cost of the EDFA through a mass-production. Accordingly, there is a need for remedy about these problems associated with the EDFA.
Recently, as broad-band light sources are widely used in the wavelength division multiplex (WDM) passive optical network (PON) field, a semiconductor optical amplifier (SOA) cheaper than other conventional optical devices is gaining its popularity as a broad-band light source.
The semiconductor optical amplifier (SOA) can be used as both an amplifier and an optical element in an optical switch, a wavelength converter, an all-optical logic circuit, a signal reproducer, and transmitter/receiver. As such, the SOA has various structures according to its application.
FIG. 1 is a schematic view of a conventional traveling semiconductor optical amplifier (SOA) 100 having a function of amplifying the light inputted through an input terminal and then outputting the amplified light through an output terminal. The traveling SOA 100 comprises an active layer 101 serving as a gain area, an under-cladding and over-cladding layers 102 and 103 formed on the lower and upper surfaces of the active layer for limiting an optical route into the active layer 101, and antireflection (AR) coating layers 104 and 105 having reflectivities of R1 and R2, respectively. Arrows in FIG. 1 represent an amplified spontaneous emission (ASE) of light.
In order to use the SOA as a broad-band light source, it is desirable to use a reflective SOA, of which light is inputted to and outputted from one surface so as to generate the amplified spontaneous emission (ASE) of light.
FIG. 2 is a schematic view of a conventional reflective semiconductor optical amplifier (SOA) 200. The reflective SOA 200 comprises an active layer 201 serving as a gain area, an under-cladding layer 202, an over-cladding layer 203, an AR coating layer 204 having reflectivity of R3, and a high reflection coating layer 205 having reflectivity of R4. The reflective SOA 200 differs from the traveling SOA 100 shown in FIG. 1 in that the AR coating layer 204 is formed at one surface of the active layer 201 of the reflective SOA 200, and the high reflection coating layer 205 is formed at the other surface of the active layer 201 of the reflective SOA 200. As such, in the reflective SOA 200, light inputted through one surface of the active layer 201 is not outputted through the other surface of the active layer 201, but reflected and amplified by the active layer 201 one more time and then outputted through the input terminal.
In order to maintain a low gain ripple, it is required to drop the reflectivity of the AR coating layer 204 of the conventional reflective SOA 200. FIG. 3 is a graph illustrating the variation in the gain ripple according to the variations in the gain and the reflectivity of a cross section of the amplifier. It is noted that in order to obtain a gain ripple of not more than 10% (0.5 dB) at a high gain, a low reflectivity must be maintained. For example, in case that the amplifier has a gain (5) of 30 dB, multiplication of the reflectivities R3 and R4 at both surfaces of the amplifier must not be more than 1×10−8 to obtain the gain ripple of less than 0.5 dB. Accordingly, if it is assumed that the reflectivity of the high reflection coating layer is more than 30%, it can be concluded that the reflectivity at the output terminal must not be more than 1×10−8. However, since the reflectivity obtained through a general process of manufacturing a semiconductor optical amplifier using the angled structures of a waveguide, a window, and antireflection coating layers being approximately 1×10−5, the gain ripple is considerably high, as shown in FIG. 4. As a result, when the amplifier is operated in the area of a very low gain, the amplifier can obtain low gain ripple characteristics easily. However, in order to obtain low gain ripple characteristics in an area of high gain, the reflectivity of the output terminal of the amplifier must be lowered drastically.